Answers

OK:

Frag grenades: Normally, troops that are in cover always strike first
in close combat. Frag grenades stop this, so that you fight
simulatneously.

Krak grenades: Can only be used against vehicles and walkers. Roll to
hit as normal, exchanging all your attacks for a single attack with a
krak grenade. Basically, they allow you to make a Strength 6 attack
against a vehicle.